The present invention relates to a control system for a motor vehicle having a continuously variable transmission and an engine.
There is known a control system for a motor vehicle having a continuously variable transmission and an engine. Such control system is disclosed in laying-upen Japanese Patent application 58-160661. In this known control system, horse power required for engine is determiend as a function of depression degree of an accelerator pedal associated with the engine, and an output torque and an engine speed which accomplish the horse power required are determined as a target output torque and a target engine speed, respectively. Reduction ratio of the continuously variable transmission is adjusted via feed-back control toward a target value where the target engine speed is accomplished and amount of intake air (or fuel) supplied to the engine is adjusted, via feed-back control, toward a target value large enough to cause the engine to produce the target output torque.
The above mentioned known control system poses a problem during transient operation period after depression degree of accelerator pedal changes rapidly. Such rapid change in depression degree of accelerator pedal causes a rapid change in intake air amount supplied to engine and a less rapid change in engine speed. The sluggish response of engine speed during such transient period results from the fact that the engine speed is varied by shifting continuously variable transmission and actual shifting in reduction ratio takes a considerable time when the deviation is large. As a result, the engine cannot follow a best fuel consumption rate operation line (which may also be called as a best fuel economy operation line) and operates off the track of this operation line. This is more specifically explained referring to FIGS. 6 and 7. Assume now that horse power expressed in terms of depression degree of accelerator pedal is to change rapidly from a point on one iso-horse power curve Pa to another point on another iso-horse power curve Pb (i.e., in the case where operation state of engine is reqired to change from an operation point A to another point B). This causes a change in target engine speed from a value Na to another value Nb and a change in target intake air amount from Ga to Gb (see FIG. 6). As shown by fully drawn line in FIG. 7, actual intake air amount follows the change in the desired intake air amount and reaches the target value (Gb) with little delay, but, since actual shifting taking place in continuously variable transmission is not quick, a considerasble time is taken for the engine to reach a target value (Nb). Thus, eEngine traces operation line as illustrated by broken line in FIG. 6 which is deviated from the best fuel consumption rate operation line. As a result, fuel is consumed than necessary increases. The same problem arises in the case when horse power is to change rapidly from a point on iso-fuel consumption rate curve Pb to another point on iso-fuel consumption rate curve Pa.
The present invention contemplates to solve the above problem and provides a control system for a motor vehicle having a continuously variable as ansmission and an enginewhereby an associated engine keeps on operating on a best fuel consumption rate operation line even if there takes place a rapid change in horse power demand, which may be expressed in terms of a change in depression degree of accelerator pedal.